The present invention relates to a treating agent effective for making harmless and immobilizing noxious metals contained in fly ash discharged from an urban dust incinerator plant and a method for the immobilizing treatment using it.
Conventionally, fly ash formed from an urban dust incinerator plant containing noxious metals has been carried out of the incinerator after mixed with bottom ash of the incinerator and filled up in a final reclaiming site or has been sprayed by water to prevent scattering of ash and then carried out in most cases.
In Japan, the fly ash of an urban dust incinerator has been made to be treated as a particularly controlled general waste after "the law related to the treatment and the cleaning of waste" was amended on October, 1991 and enforced on July, 1992 and thus it should be filled up after treated to be harmless.
However, a large amounts of lime (CaO) and calcium hydroxide (Ca(OH).sub.2) are blown into the flue in an excess of the theoretical amount to make harmless the detrimental gases discharged such as hydrochloride (HCl) and sulfur oxide (SOX) and thus unreacted lime and calcium oxide, reaction product thereof with gases, calcium chloride (CaCl.sub.2) and gypsum (CaSO.sub.4) are collected in the electric dust collector and the bag filter together with fly ash formed from an urban dust incinerator plant and discharged in a mixed condition with the fly ash. Therefore, such fly ash collected contains a large amount of detrimental metals and shows as high an alkalinity as a pH of not lower than 12, generally 12.0 to 13.5 and has a property of re-eluting lead (Pb), cadmium (Cd), zinc (Zn) and copper (Cu) and thus is difficult to be treated to be made harmless and immobilized by conventional technologies.
Though general methods include at present the cement solidification method by using common portland cement and special cements, the chemical treatment method by using sulfur chelate compounds or the acid extraction method in which the detrimental metals are extracted from fly ash with an acid, each methods involve various problems including secondary pollution and insufficient treatment for making harmless and stabilizing. Thus, the problems in the treatment of fly ash formed from an urban dust incinerator plant containing detrimental metals for making harmless and immobilizing have not been solved at present.
For solving the various problems, disclosed methods include, for example, those shown in Japanese Laid-open Patent Publication No. 129195 of 1985 and Japanese Patent Publication Nos. 19716 of 1982 and 61710 of 1992. However, the disclosure in Japanese Laid-open Patent Publication No. 129195 of 1985 aims an enhancement in cured strength, while Japanese Patent Publication No. 19716 of 1982 aims an inhibition of diffusion of detrimental metals by using cement as the base and excluding the factors preventing cure of the cement by using calcium hydroxide (Ca(OH).sub.2), aluminum sulfate (Al.sub.2 (SO.sub.4).sub.3) and sodium carbonate (Na.sub.2 CO.sub.3) and curing it to prepare cured products such as bricks and blocks and lowering contact of it with eluted water to inhibit diffusion of detrimental metals. They are thus not treatments for immobilizing detrimental metals by a direct reaction with chemicals.
Japanese Patent Publication No. 61710 of 1992 discloses a method for the treatment of residual ash and fly ash formed from a dust incinerator plant in which they are fixed with phosphoric acid (H.sub.3 PO.sub.4) and calcium hydroxide. However, in this patent, the mixing ratio of residual ash to fly ash (3 to 49:1) is considered to be important and it is required to mix it with calcium hydroxide previously before spraying a water-soluble phosphate source and the amount to be mixed is required to be adjusted according to the calcium hydroxide content in fly ash. Thus, it is highly dependent on pH as seen in Examples of EPA test (optimum pH=7.5 to 12.0). A fixing treatment by the method is difficult by a consideration on the solubility of detrimental metals other than Pb and Cd disclosed there such as Cu and Zn depending to the pH. In Japan, it is regulated to separate residual ash from fly ash and to discharge them each separately by law. Hence, residual ash and fly ash cannot be mixed together for the treatment to make it harmless and fly ash showing a high alkalinity should be treated to be made harmless and immobilized alone. Thus, the treating method according to Japanese Patent Publication No. 61710 of 1992 cannot be adopted.
The subject of the present invention is to solve various problems in the conventional technologies for the treatment of detrimental metals contained in fly ash formed from a dust incinerator plant and to provide a treating agent for making it harmless and immobilize it and a method for the immobilizing treatment by sealing and fixing it surely and immobilizing it.
We, inventors, have investigated and have found that fly ash formed from a dust incinerator plant can be treated to be harmless and immobilized by a direct reaction by using a treating agent for making harmless and immobilizing it containing specific starch, sodium silicate or a hydrogen phosphate compound and allophane and a treating agent for making harmless and immobilizing it in wider sense further containing at least one selected from the group consisting of cement, gypsum or fly ash formed from a coal thermoelectric power plant in the above-mentioned treating agent for making harmless and immobilizing it (special cement solidifier in the case containing cement) to complete the present invention.